ci: tidy up benchmarks a little

[davidhewitt]

Codspeed has a new feature to ignore benchmarks which are volatile, so we can resume running them here (and collecting their history) but not get annoying messages on PRs.

Also, some other tiny tidy ups at the same time:

• Use std::hint::black_box rather than the one from criterion.
• Remove some remaining uses of gil-refs 👀
• Be a little bit more consistent about just black-boxing the input and returning output.

[codspeed-hq]

CodSpeed Performance Report

Merging #3986 will improve performances by 22.95%

_{Comparing davidhewitt:benchmarks-tidy (2d790dc) with main (009cd32)}

Summary

⚡ 2 improvements
✅ 64 untouched benchmarks

🆕 11 new benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:benchmarks-tidy	Change
🆕	mapping_from_dict	N/A	383.3 ns	N/A
⚡	extract_float_downcast_fail	548.3 ns	493.9 ns	+11.02%
🆕	extract_float_downcast_success	N/A	157.2 ns	N/A
🆕	extract_float_extract_success	N/A	344.4 ns	N/A
🆕	extract_int_downcast_fail	N/A	218.3 ns	N/A
🆕	extract_int_downcast_success	N/A	627.2 ns	N/A
🆕	extract_int_extract_success	N/A	533.3 ns	N/A
⚡	extract_str_downcast_fail	300.6 ns	244.4 ns	+22.95%
🆕	list_via_downcast	N/A	157.2 ns	N/A
🆕	list_via_extract	N/A	187.2 ns	N/A
🆕	not_a_list_via_downcast	N/A	244.4 ns	N/A
🆕	sequence_from_list	N/A	355.6 ns	N/A
🆕	sequence_from_tuple	N/A	296.7 ns	N/A

[davidhewitt]

As a CI-only change I'm going to proceed to merge this as a tidy-up; I'll remain vigilant for any unwanted volatility and disable corresponding benchmarks on the codspeed dashboard where needed.